An interface circuit of the full-duplex type, designed to facilitate the separation of balanced incoming and outgoing signals in such a transceiver, has been described in an article entitled "A NOVEL HIGH-SPEED INTERFACE CIRCUIT SAVING WIRING EQUIPMENT" by Wilhelm Wilhelm, Karl-Reinhard Schon and Hans Kaiser, IEEE Journal of Solid-State Circuits, Vol. SC-13, No. 3, June 1978. According to that article, signal separation is achieved with the aid of a resistor network which can be realized in integrated circuitry also incorporating a number of associated transistors. The network subtracts the locally generated outgoing signal, available at the output of a driving circuit, from a mixture of outgoing and incoming signals present at the distal end of a line-terminating impedance which matches the characteristic impedance of the line and is constituted by a pair of resistors inserted in its two conductors. The latter resistors are part of the integrated circuitry, as are a pair of load resistors through which twin final transistors of the line driver are connected to their power supply.
The known circuit arrangement has the drawback that the resistor network strongly attenuates the outgoing signal so as to require additional amplification at both ends of the line. The incorporation of the line-terminating resistors in the integrated circuitry limits its adaptability to lines of different characteristic impedances. Moreover, an attempt to use that circuitry with an unbalanced line would give rise to an unfavorable signal-to-noise ratio.